I only had a vague idea on how the chains could be used until I built my 1/3 scale 6-Pounder and built the chains to scale as well.
I had read that the chains would be used to prevent the wheels from rotating. This could be used if you were positioned just over the rise of a hill to prevent your piece from recoilling all the way to the bottom of the hill after each shot. If set up on a down slope, it would stop the cannon from rolling down the hill and becoming your enemy's cannon. In other situations, securing the lock chains to the wheels could be used simply to shorten the recoil distance.
When I had a chance to play with the 1/3 scale and experiment with the lock chains, I found there were a great many ways they could be used.